No, it's not gravity, it's something else. Something mysterious. Something barely perceptible that is tugging at the probe with about 10 million times weaker force than gravity. Potentially a "new force of nature" (their words, not mine).

Most of the obvious causes, including malfunction or gravity, have been ruled out, reports the Telegraph:

Scientists initially suspected that gas escaping from tiny rocket motors aboard the probes, or heat leaking from their nuclear power plants might be responsible. Both have now been ruled out. The team says no current theories explain why the force stays constant: all the most plausible forces, from gravity to the effect of solar radiation, decrease rapidly with distance.

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Scientists tracking the distant probe (now some seven billion miles from Earth), say that the probe's speed, presently 27,000mph, is being reduced by the force by about 6mph per century. Not much, but entirely noticeable and worthy enough that scientists like Dr. Duncan Steel, of Salford University, who is speculating this cosmic tug could question whether we know enough about gravity, the universe, and everything. [Telegraph]